1. Field of the Invention
The present invention relates to an electromagnetic valve for controlling the flow of a metal or metal alloy in liquid phase in a pipe under load, comprising a tubular body made of a material permeable to the magnetic field and at least one polyphase field coil arranged around the tubular body in order to create a magnetic field sliding lengthwise along the tubular body.
2. Discussion of Background Information
In the field of metallurgy, for example, in an industrial casting equipment or in equipment for covering iron and steel products with a coating of metal or metal alloy, such as hot galvanization equipment, or in other applications, it is often necessary to be able to control a flow of metal or metal alloy in liquid phase. In this respect, either the metal or the metal alloy is in molten state following a controlled increase of its temperature, or the metal or metal alloy is normally liquid at ambient temperature which is the case, for example with mercury. In order to control a flow of liquid metal or metal alloy, it is usual to use electromechanical or hydromechanical systems such as nozzles with slide valve, stopper rods, etc. These systems involve considerable investment and relatively high maintenance costs due to the presence of movable mechanical elements in these systems.
It is for this reason that electromagnetic valves have been proposed for some time which do not comprise any movable mechanical elements for controlling the flow of a metal or metal alloy in liquid phase in a pipe under load. The electromagnetic valves of his patent operate in accordance with a principle similar to that of a linear motor. The role of the mobile armature is performed by the metal or metal alloy whose flow is to be controlled. In these electromagnetic valves, the polyphase field coil is disposed and electrically connected so that the magnetic field that it generates propagates in counter-flow with respect to the normal flow direction of the liquid metal or metal alloy in the pipe under load. In other words, the magnetomotive force generated by the polyphase field coil and applied to the liquid metal in the pipe opposes the force due to the hydrostatic pressure of the liquid metal in the pipe. By adjusting the intensity of the current in the polyphase coil, it is possible to regulate the flow rate of the liquid metal or metal alloy in the pipe. The greater is the intensity of the current in the polyphase coil, the lower the flow rate of the liquid metal or metal alloy flowing through the electromagnetic valve. Theoretically, by using a sufficiently intense current, it is possible to stop the flow of the liquid metal or metal alloy arriving at the valve. However, the intensity of the current necessary for stopping the flow of metal liquid or metal alloy is relatively great. Therefore, because the electrical power necessary for maintaining the electromagnetic valve in the "closed" state is great, it has proved difficult to obtain a complete reliable stop of the flow of liquid metal or metal alloy.
In order to obtain a complete stoppage of the flow of liquid metal or metal alloy, it has been proposed to partially close the outlet end of the tubular body of the electromagnetic valve by a transverse wall presenting an outlet orifice offset or off-center with respect to the longitudinal axis of the tubular body. Although such an arrangement effectively insures complete stop of the flow of liquid metal or metal alloy, the intensity of the current necessary for that purpose remains relatively high. In addition, when the electromagnetic valve is "opened", the transverse wall with its off-center outlet orifice causes disturbances (turbulence) and pressure drops in the flow of liquid metal or metal alloy which, in certain applications, may be unacceptable.